1. Field of the Invention
The present invention relates to an improvement of a butterfly valve, more particularly, to butterfly valve resistant to corrosive gases having a high temperature of more than 70.degree. C.
2. Description of the Prior Art
To improve the anticorrosive performance of butterfly valves, it has been well known to provide a coating layer of fluororesin or the like on the fluid contacting surfaces of the valve disk as well as the valve body.
The present inventor has previously developed an anticorrosive butterfly valve with the valve disk coated with polytetrafluoro ethylene (PTFE) and with the valve seat ring (made of natural or synthetic rubber), extending along the inner surface of the valve body, lined with polyvinylidene difluoride (PVDF). Since the valve seat ring is composed of an inner layer of PVDF and an outer layer of rubber or its equivalent, the contact between the valve disk and the valve body can be considerably improved (refer to Japanese Examined Patent Publication (Kokoku) No. 56-31464).
However, this type of butterfly valve has been found to have several drawbacks. Namely, while the gas permeability of the PVDF lining the rubber layer of the valve seat ring is very low, e.g., 9.32.times.10.sup.-10 cc.multidot.cm/cm.sup.2. sec.cmHg for gaseous chlorine at 80.degree. C., and, thus, in use for pipe lines conveying, for example, gaseous chlorine of a high temperature of more than 70.degree. C., almost no dry gaseous chlorine will permeate through the lining layer to the rubber layer, preventing corrosion of the rubber layer by the gaseous chlorine, the moisture permeability of the PVDF is considerably high e.g., 230.times.10.sup.-10 cc.cm/cm.sup.2 .multidot.sec.multidot.cmHg for gaseous chlorine is at 40.degree. C. and 516.times.10.sup.-10 cc.multidot.cm/cm.sup.2 .multidot.sec.multidot.cmHg at 80.degree. C. Thus, the moisture contained in the gaseous chlorine tends to permeate through the lining layer into the elastic layer.
If the elastic layer of the valve seat ring is made from material with no moisture permeability or lower moisture permeability than that of PVDF, the moisture contained in the gaseous chlorine tends to permeate into the clearance between the PVDF layer and the elastic layer after a long period of valve use. This results in the detaching of the adhesive between the PVDF layer and the elastic layer and in the accumulation of the permeated moisture in the resultant space. Thus accumulated moisture pushes up the PVDF layer. Thus, during on-off operation of the valve, the PVDF layer tends to be broken by repeated frictional contact against the valve disk. Accordingly, corrosion of the elastic layer by gaseous chlorine is caused and the sealing function of the valve is lost.
On the other hand, if the moisture permeability of the elastic layer of the valve seat ring is higher than that of the PVDF layer, the moisture of the gaseous chlorine permeates through the PVDF layer and the elastic layer to accumulate in the clearance between the elastic layer and the valve body. This results in the undesired deformation of the elastic layer and reduced sealing function of the valve.